Polymer-matrix composites offer unique combinations of properties and are useful in a wide range of applications. Such composites may be fabricated utilizing either thermosetting or thermoplastic polymer matrix materials with a variety of particulate or fibrous fillers or reinforcements. It is generally advantageous to have strong adhesion between the polymer matrix material and the surfaces of the various particulate or fibrous substrates and there is considerable art related to substrate finishes and other treatments to optimize adhesion to polymer matrices. For example, in the production of long-fiber reinforced composites, improved adhesion between the polymer matrix and the fiber reinforcement leads to increased material performance. Good adhesion is particularly important where failures are likely to occur by delamination or by other adhesive failure modes.
As described in, for example, U.S. Pat. Nos. 5,840,238, 6,310,121, and 6,525,125, the disclosures of each of which are incorporated herein by reference, polymers generated by olefin metathesis processes are attractive as composite matrix materials. Of particularly beneficial use are the polymers generated by the ROMP of cyclic olefins. The low viscosity of cyclic olefin resin formulations and the ability to control ROMP kinetics (e.g., U.S. Pat. Nos. 4,708,969 and 5,939,504, the disclosures of both of which are incorporated herein by reference) facilitate composite processing and manufacture, and the corrosion resistance and high toughness of ROMP polymers leads to good composite durability. Additionally, certain properties of ROMP polymers, e.g., mechanical strength and stiffness, heat distortion temperature and solvent resistance, can be further enhanced by crosslinking induced via thermal treatment (e.g., U.S. Pat. No. 4,902,560, the disclosure of which is incorporated herein by reference) or chemically by addition of peroxides (e.g., U.S. Pat. No. 5,728,785, the disclosure of which is incorporated herein by reference).
Commercially important ROMP resin formulations are generally based on readily available and inexpensive cyclic olefins such as dicyclopentadiene (DCPD), norbornenes, cyclooctadiene (COD), and various cycloalkenes. However, in contrast to traditional resin systems (e.g., epoxy, acrylate, urethane, and polyester resins) based on polar functional group chemistries, these nonpolar ROMP resins have poor intrinsic adhesion to the relatively polar surfaces of common carbon, glass, or mineral fillers and reinforcements. The addition of various silanes to such resin formulations for improvement of electrical and mechanical properties of ROMP polymers is described in U.S. Pat. Nos. 5,840,238, 6,001,909, and 7,339,006, the disclosures of each of which are incorporated herein by reference. Many widely used commercial silanes do not give optimal properties with ROMP polymers, however, and the greatest enhancements are only obtained when the silanes comprise groups with high metathesis activity (the relative reactivity of various metathesis active groups is described in J. Am. Chem. Soc., 2003, 125, 11360-11370).
Polymers generated by ROMP are particularly well-suited for casting of molded parts and infusion of resin-glass and resin-wood composites, as non-limiting examples. According to one method process, the cyclic olefin monomer is blended with appropriate additives and fillers, and then mixed with an olefin metathesis catalyst. The initial resin mixture is typically a low-viscosity liquid, allowing for a wide range of resin infusion and casting techniques. As the polymerization proceeds, the resin first “gels” (increases in viscosity such that it no longer flows freely) and then “cures” as the resin reaches peak monomer conversion. The kinetics of the rate of gel and cure of olefin metathesis polymerizations depend on monomer, catalyst, and temperature.
When manufacturing articles using olefin metathesis polymerization, any pouring or infusion of catalyzed resin must be complete before the resin viscosity increases to the point that the resin no longer flows to fill the mold under the manufacturing conditions. Pouring or infusion of highly viscous (pre-gelled) or gelled resin may lead to inclusion of trapped air, or produce other defects or conditions that decrease the mechanical properties or visual appearance of the manufactured part. It would, therefore, be desirable to control the gel formation process, in particular to delay the onset of viscosity increase and the onset of the resin gel and cure states, through the use of a gel modification agent. Once the pour or infusion is complete, it would be further advantageous for the onset of polymerization to begin within a reasonable time after the mold is filled, and to proceed at a desirable rate of cure.
The time during which the liquid monomer/catalyst mixture can be worked after the monomer and catalyst is mixed is called the “pot life” of the polymerization reaction mixture. The ability to control the “pot life” becomes even more important for the molding of large parts and to achieve defect-free infusion of porous materials. It would be particularly useful to be able to control the gel formation process, especially the onset of the gel state, of catalyzed ROMP reactions when such large parts are to be produced, or when defects arising from viscosity build-up are to be reduced or eliminated.
Certain limited types of gel modification agents for olefin metathesis polymerizations have been disclosed. For example, U.S. Pat. No. 5,939,504 discloses the use of phosphines, pyridines, and other Lewis bases as gel modifiers. While useful, the effect of such gel modifiers in ROMP reactions can be difficult to control, particularly where relatively small changes in the onset of polymerization are desired. For example, while the addition of small amounts of tributylphosphine, a commercially attractive additive because of its low cost, may produce no noticeable change in pot life, adding a slightly greater amount may overshoot the desired effect by creating a significantly longer delay in the onset of polymerization than desired. From a practical perspective, the inability to finely control the gel formation process makes these gel modifiers less useful in the manufacture of articles of large or varying dimensions. Certain gel modifiers, such as phosphines, also oxidize quite quickly in resin thereby decreasing the ability of the modifier to extend the pot life. Resin compositions relying on phosphine compounds for gel modification, therefore, cannot be stored for any appreciable length of time without reformulation with fresh gel modification additive.
Although acting as activators in some systems (e.g., U.S. Pat. Nos. 4,380,617 and 4,049,616), active oxygen containing compounds, including hydroperoxides, are generally considered to have a negative impact on metathesis catalyst performance. Olefins intended for use in metathesis reactions are often chemically treated (e.g., U.S. Pat. No. 5,378,783) or pre-treated with an adsorbent such as alumina or zeolites (e.g., U.S. Pat. Nos. 7,700,698; 4,943,397; and 4,584,425) to reduce the concentration of oxygen-containing impurities such as hydroperoxides. For example, U.S. Pat. No. 4,584,425 shows that hydroperoxide compounds have a significant negative impact on the ROMP of DCPD with a two part tungsten metathesis catalyst and U.S. Pat. No. 7,576,227 teaches that it is advantageous to remove hydroperoxides and other catalyst poisons to improve cross metathesis turnover number when using ruthenium alkylidene catalysts.
Hydroperoxide additives have been suggested as post-polymerization radical crosslinking initiators for ROMP polymers (e.g., U.S. Pat. Nos. 7,025,851 and 7,476,716). However, U.S. Pat. No. 5,728,785 specifically shows that ROMP of dicyclopentadiene fails in the presence of 1 wt. % (relative to dicyclopentadiene) of tert-butyl hydroperoxide, a level typically useful to effect post-polymerization cross-linking. Others teach that additives used in ROMP formulations should not contain hydroperoxide functionalities, so as to avoid adverse interactions with metathesis catalysts (e.g., U.S. Pat. Nos. 6,323,296 and 6,890,650, the disclosures of which are incorporated herein by reference).
Despite the advances achieved in the art, particularly in the properties of olefin metathesis polymers and their associated applications, a continuing need therefore exists for further improvement in a number of areas, including the adhesion of olefin metathesis compositions, in particular, ROMP compositions, to substrate materials, especially the wide variety of existing substrate materials that have been used with traditional resin systems, and the use of certain gel-modifiers to control the gel formation process of polymerizing ROMP compositions.